Epidemiological and experimental studies have implicated high dietary fat in the increased incidence of mammary cancer. Experimental systems have been established for assessing fatty acid effects on the growth of cells from normal tissue and carcinogen induced mammary tumors of rats. The studies show that all unsaturated fatty acids present in the gland in appreciable amounts promote cell growth. Both normal and tumor cells are inhibited by saturated fatty acids. Maximum growth rate of tumor cells is obtained with oleic acid while normal cells grow best in the presence of linoleic or linolenic acid. Both normal and tumor cells may be able to recruit fatty acids needed for growth from neighboring tissue. Perphenazine treatment of animals produces an increase in the relative amount of unsaturated fatty acids in the mammary gland but not in omental fat. This appears to be effected by the action of prolactin. Maps of fatty acid types at various distances from growing mammary tumors have been made. These suggest a concentration of unsaturated relative to saturated fatty acids closest to the tumors.